In recent years, with the age of highly computerized societies, communication machines and equipment for satellite broadcasting, mobile phones, etc., tend to be digitized, and signal processing tends to be quicker. These communication machines, etc., have printed wiring boards constituted of composite materials composed of materials such as a reinforcing material, a resin, a modifier, a filler, and the like. Further, glass fibers are widely used as a reinforcing material for their peripheral plastic members. Conventionally, E glass is known as a commercially produced glass fiber of this kind.
When an alternate current is flowed in glass, generally, the glass absorbs energy with regard to the alternate current and absorbs it as heat. The dielectric loss energy to be absorbed is in proportion to a dielectric constant and a dielectric tangent which are determined by the components and the structure of the glass, and is shown by the following expression.W=kfv2×ε tan δ
in which W is a dielectric loss energy, k is a constant, f is a frequency, v2 is a potential gradient, ε is a dielectric constant, and tan δ is a dielectric tangent.
The above expression shows that with an increase in the dielectric constant and the dielectric tangent, or with an increase in the frequency, the dielectric loss increases.
E glass, for example, has a dielectric constant of 6.7 and a dielectric tangent of 12×10−4 at a frequency of 1 MHz at room temperature, and a printed wiring board using E glass is insufficient for complying with demands of a higher density and a higher processing speed. There are therefore desired glasses having a lower dielectric constant and a lower dielectric tangent than the E glass. Among them is a glass called D glass. D glass is, for example, a glass having a composition containing, by weight %, 75.3% of SiO2, 20.5% of B2O3, 0.4% of MgO, 0.6% of CaO, 0.6% of Li2O, 1.1% of Na2O and 1.5% of K2O. For example, it has a dielectric constant of 4.3 and a dielectric tangent of 10×10−4 at a frequency of 1 MHz at room temperature.
However, D glass has the following defects. Since it has poor meltability and is liable to cause striae and foams, a glass fiber frequently breaks during its spinning step, and it is poor in productivity and, workability. Further, since it has a very high spinning temperature, the lifetime of a furnace is caused to decrease. Moreover, there is another problem that since D glass has poor water resistance and poor adhesion to a resin, it is liable to peel from a resin of a printed wiring board so that no high reliability can be obtained when it is used for a printed wiring board.
JP-A-6-219780 discloses a low-dielectric-constant glass comprising, by weight %, 50.0 to 65.0% of SiO2, 10.0 to 18.0% of Al2O3, 11.0 to 25.0% of B2O3, 6.0 to 14.0% of MgO, 1.0 to 10.0% of CaO and 0 to 10.0% of ZnO, the content of MgO+CaO+ZnO being 10.5 to 15%. In the above glass, it is intended to decrease a spinning temperature in order to improve productivity particularly by incorporating at least 6% of MgO and adjusting CaO+MgO+ZnO to at least 10.5%. Since, however, MgO, a component which is highly liable to undergo phase separation and give a high dielectric tangent, is incorporated in an amount of 6% or more, no sufficient water resistance can be obtained, and the dielectric tangent that can be achieved by the glass is approximately 10×10−4.
The present inventors have already proposed, by JP-A-8-333137, a glass comprising, by weight %, 50 to 60% of SiO2, 10 to 20% of Al2O3, 20 to 30% of B2O3, 0 to 5% of CaO, 0 to 4% of MgO, 0 to 0.5% of Li2O+Na2O+K2O and 0.5 to 5% of TiO2. The above glass retains a low dielectric constant and a low dielectric tangent and at the same time shows excellent properties with regard to workability and productivity during a spinning step. However, it shows a relatively high volatilization amount of B2O3, and during the spinning of a glass fiber, the glass fiber sometimes undergoes breaking due to B2O3-induced adhering around a bushing nozzle. It has been therefore still desired to further improve the fiber in workability and productivity.
When the above glass fiber is used for a printed wiring board, generally, a plurality of fabrics made of the glass fiber are laminated, and the formed laminate is used for a printed wiring board.